Re: [PATCH v2 0/3] Clarify abstract scale usage for power values in Energy Model, EAS and IPA

[Quentin Perret <qperret@google.com>]

On Thursday 15 Oct 2020 at 15:40:16 (+0200), Rafael J. Wysocki wrote:
> On Thu, Oct 15, 2020 at 12:22 PM Daniel Lezcano
> <daniel.lezcano@linaro.org> wrote:
> >
> > On 15/10/2020 11:00, Lukasz Luba wrote:
> >
> > [ ... ]
> >
> > >> There is the SCMI and the DT. Because there are two sources where it is
> > >> impossible to know if they are using the same units, we are stuck to
> > >> ensure a consistency for the kernel.
> > >>
> > >> The platform should use:
> > >>   - the SCMI only (scaled or real)
> > >>   - the DT only (real)
> > >>   [ - the firmware file only (scaled or real) ]
> > >>
> > >
> > > Do you mean by SCMI - registration using em_dev_register_perf_domain() ?
> >
> > It was high level description, but yes, I guess it is the case.
> >
> > >> As it is not possible to know if they are scaled or real, there is no
> > >> choice except making them mutually exclusive.
> > >
> > > So you propose a bit more restriction in registration EM, to not get
> > > lost in the future. I also have these doubts. Let's consider it and
> > > maybe agree.
> > >
> > > I've recommended Qcom to use em_dev_register_perf_domain() when they
> > > have this obfuscated power values. Then any developer in the future
> > > who wants to add EM for a new device on that platform, should use the
> > > em_dev_register_perf_domain().
> > >
> > > In this case the flag in EM that you have proposed makes sense.
> > > We probably need an argument 'bool abstract_scale' in the
> > > em_dev_register_perf_domain(..., bool abstract_scale)
> > > as a source of information.
> >
> > I was suggesting to add a flag to the em_perf_domain structure giving
> > the source of the power numbers.
> >
> > So if the IPA is having the 'sustainable-power' set in DT but the
> > em_perf_domain is flagged with power number coming from SCMI, then they
> > will be incompatible, the thermal zone will fail to register.
> >
> >
> > > We would allow to co-exist em_dev_register_perf_domain(..., false)
> > > with dev_pm_opp_of_register_em() EM devices.
> > >
> > > Is it make sense?
> >
> > Well, it does not change my opinion. We should assume the energy model
> > is always milliwatts. If the SoC vendors find a way to get around with
> > bogoWatts, then good to them and up to them to deal with in the future.
> 
> That sounds fair enough, but it also means that any kernel patches
> using power units different from milliwatts for the EM should be
> rejected in the future, doesn't it?
> 
> And the existing code using different power units for the EM (if any)
> should be updated/fixed accordingly, shouldn't it?
> 
> Otherwise I don't see now this can be regarded as a hard rule.

Sorry, jumping late in the discussion :)

To add a bit of background to this, it's been the plan from the very
beginning to make PM_EM use an abstract scale. The only reason it was
not merged like that is because the first version only worked for CPUs,
and IPA was using a totally different source for other devices. So we
had no choice but to specify PM_EM in mW to keep things compatible and
allow to transition IPA. But that is no longer true, so I'm in favor of
evolving PM_EM where it was supposed to be to begin with.

IMO, the only thing the kernel cares about is consistency across power
numbers, but not about the exact unit. And I agree with Rafael, we have
code paths in the kernel that feed data in PM_EM but _cannot_ guarantee
mW, SCMI being a prime example, so I don't think it is reasonable to
mandate that.

Having that properly documented + an 'abstract_scale' parameter in
dev_pm_opp_of_register_em() (or even a unit, which could be bogo-watts)
should work IMO. What is the concern with this approach?

Thanks,
Quentin